For example, as in the above-mentioned Patent Literatures, a hologram recording/playing system is known, which performs data recording by an interference pattern of signal light and reference light. With this hologram recording/playing system, at the time of recording, signal light provided with spatial-optical modulation according to the recording data (e.g. optical intensity modulation), and reference light that is separate from the signal light herein, are irradiated onto the hologram recording medium, and an interference pattern thereof is formed on the hologram recording medium, thereby performing data recording.
Also, at the time of playing, the reference light is irradiated onto the hologram recording medium. Thus, by the reference light being irradiated, diffracted light according to the interference pattern formed on the hologram recording medium is obtained as described above. Thus, playing light (playing signal light) in accordance with the recording data is thus obtained. The playing light thus obtained is detected with an image sensor such as a CCD (Charge Coupled Device) sensor or CMOS (Complementary Metal Oxide Semiconductor) sensor, whereby the recording data is played.
Now, as a hologram recording playing system, similar to a conventional optical disc recording/playing system such as a CD (Compact Disc) or DVD (Digital Versatile Disc), there is consideration being given to recording data along a track formed on the recording medium. That is to say, by performing recording/playing position control such as a tracking servo which handles tracks, similar to the case of a conventional optical disc, data recording is performed at appropriate positions on the disc.
An example of a configuration of the hologram recording medium used in the case of performing such recording/playing position control will be described using a cross-sectional configuration diagram in FIG. 32.
In FIG. 32 shows a configuration example of a hologram recording medium 1000 of a reflective type that has a reflective film.
As shown in the diagram, the hologram recording medium 100 has a recording layer (106), whereupon recording of the hologram is performed with the interference pattern of the above-described signal light and reference light, and a position control information recording layer, whereupon recording of address information and the like for position control has been performed with an uneven cross-sectional configuration on a substrate 110, are each formed separately.
Specifically, the hologram recording medium 100 has, in sequence from the upper layer, a cover layer 105, recording layer 106, reflective film 107, intermediate layer 108, reflective film 109, and substrate 110.
Laser light (the above-described reference light) for playing the hologram at the time of playing is irradiated onto the reflective film 107 formed on a layer under the recording layer 106, and upon a playing image according to the hologram recorded on the recording layer 106 being obtained, this is provided to be returned to the device side as reflected light.
Also, a track for guiding the recording/playing position of the hologram on the recording layer 106 is formed on the substrate 110 in a spiral shape or in a concentric fashion. For example, the track may be formed by information recording of address information or the like being performed with pit rows.
The reflective film 109 formed on the layer above the substrate 100 is provided to obtain the reflected light according to the pit rows. Note that the intermediate layer 108 is an adhesive material such as a resin, for example.
The recording/playing light for performing recording/playing of the hologram on the recording layer 106 and the position control light for obtaining the reflected light from the position control information are each individually irradiated onto the hologram recording medium 100 which has a cross-sectional configuration such as described above.
Now, if we say that only one type of light is used for both the recording/playing of the hologram and the position control, there is the risk that the components corresponding to the uneven cross-sectional shape of the substrate 110 (reflective film 109) will be superimposed on the playing image of the hologram as noise, thereby deteriorating playing capabilities. Therefore, for position control with the hologram recording/playing system, the position control light for obtaining the reflected light from the position control information recording layer is separately irradiated, along with the recording/playing light of the hologram.
Also, in the case of thus irradiating the hologram recording/playing light and the separate position control light, lights having differing wavelength bands are used. The reason thereof is, in the case of using light having the same wavelength bands for position control light and recording/playing light, there is the risk that of recording layer 106 becoming photosensitized due to the irradiation of the position control light, so this is for the prevention thereof.
For example, for the hologram recording/playing light, a violet laser light having a wavelength of approximately λ=405 nm is used, and for the position control light, red laser light having a wavelength of approximately λ=650 nm is used.
Now, in order to obtain reflected light from the position control information recording layer by irradiation of the position control light, the position control light must arrive at the reflective layer 109 whereupon the uneven cross-sectional shape of the substrate 110 is reflected. That is to say, the position control light has to pass through the reflective film 107 that is formed on a layer above the reflective film 109.
On the other hand, the hologram recording/playing light has to reflect at the reflective film 107, so that the playing image according to the hologram recorded on the recording layer 106 is returned to the device side as reflected light.
With consideration for these points, a reflective film having wavelength selectivity is used for the reflective film 107, whereby the violet laser light for recording/playing reflects and the red laser light for position control passes through. Thereby, the position control light arrives at the reflective film 109 and the reflected light for position control is appropriately returned to the device side, and at the same time, the playing image of the hologram recorded on the recording layer 106 can be reflected with the reflective film 107 and appropriately returned to the device side.
Now, in the case of thus using light that is separate from the hologram recording/playing light to perform control of the recording/playing position, on the recording/playing device side, the hologram recording/playing light and the position control light are composited on the same optical axis, as shown in the following FIG. 33, and irradiates the composited light as to the hologram recording medium 100. Tracking servo control based on the reflected light of the position control light is then performed.
Thus, by performing position control based on the reflected light of the position control light while synthesizing the hologram recording/playing light and position control light on the same optical axis and irradiating this onto the hologram recording medium 100, the hologram recording/playing position is controlled at positions along tracks (pit rows) formed in the hologram recording medium 100.